Oil equalizing circuit compression mechanisms, heat source unit for freezing device, and freezing device having the same

ABSTRACT

The present invention provides an oil equalizing circuit that can improve the reliability of the oil supply to the compression mechanisms in a refrigeration system provided with a plurality of compression mechanisms. The compressor group ( 11 ) of the air conditioning system ( 1 ) is equipped chiefly with first, second, and third compressors ( 21   a  to  21   c ) and an oil equalizing circuit ( 42 ). The oil equalizing circuit ( 42 ) is equipped with the following: first, second, and third oil separators ( 24   a  to  24   c ) provided on the discharge side of the compressors ( 21   a  to  21   c ); first, second, and third oil return pipes ( 25   a  to  25   c ) connecting the oil separators ( 24   a  to  24   c ) to the intake sides of the compressors ( 21   a  to  21   c ); a communication pipe ( 26 ) for allowing the oil return pipes ( 25   a  to  25   c ) to communicate with one another; first, second, and third oil ON-OFF switching means ( 27   a  to  27   c ) provided downstream of the parts where the oil return pipes ( 25   a  to  25   c ) connect to the communication pipe ( 26 ); and first, second, and third pressure reducing means ( 28   a  to  28   c ) provided upstream of the parts where the oil return pipes ( 25   a  to  25   c ) connect to the communication pipe ( 26 ).

TECHNICAL FIELD

[0001] The present invention relates to an equalizing circuit for a compression mechanism, a heat source unit for a refrigeration system, and a refrigeration system equipped with the same. More specifically, the present invention relates to a vapor compression refrigeration system provided with a plurality of compression mechanisms for compressing the refrigerant, the refrigeration system being further provided with an equalizing circuit for equalizing the distribution of oil among the compression mechanisms and a refrigeration system heat source unit.

BACKGROUND ART

[0002] One example of conventional vapor compression refrigeration systems provided with a plurality of compression mechanisms are air conditioning systems used to air-condition buildings. This kind of air conditioning system is provided with a plurality of user units and a heat source unit capable of accommodating the heating and cooling loads of the user units. In order to enable the system to be operated in a partial load mode, the heat source unit is provided with a circuit configuration made up of a plurality of comparatively small-capacity compression mechanisms connected in parallel. The compression mechanisms are provided with an oil equalizing circuit including oil separators connected to the discharge sides of the compression mechanisms, oil return pipes for returning the oil separated by the oil separators to the compression mechanisms, and oil equalizing pipes connected between the compression mechanisms for reducing imbalances in the amount of oil in the compression mechanisms.

[0003] In the conventional oil equalizing circuit just described, the amount of oil in each compression mechanism is ensured by providing oil return pipes and oil equalizing pipes. However, there are times when air conditioning systems provided with a plurality of compression mechanisms are run in a partial load mode in which some compression mechanisms are running and some are stopped and times when such systems are run in a partial load mode in which some compression mechanisms are operated with a reduced operating load using inverter control or the like. During these various operating patterns, it is sometimes difficult to supply oil sufficiently to the compression mechanisms that are running. Thus, the reliability of the oil supply achieved with conventional oil equalizing circuits is insufficient.

DISCLOSURE OF THE INVENTION

[0004] The object of this invention is to provide an oil equalizing circuit that can improve the reliability of the oil supply to the compression mechanisms.

[0005] The compression mechanism oil equalizing circuit described in claim 1 is an oil equalizing circuit for equalizing the distribution of oil among the compression mechanisms in a vapor compression refrigeration system provided with a plurality of compression mechanisms for compressing the refrigerant. The oil equalizing circuit is provided with oil separators, oil return pipes, a communication pipe, oil ON-OFF switching means, and pressure reducing means. The oil separators are provided on the discharge side of the compression mechanisms and function to separate the oil from the gaseous refrigerant. The oil return pipes connect each oil separator to the intake side of its respective compression mechanism and allow the oil separated by the oil separators to be delivered to the intake sides of the compression mechanisms. The communication pipe is connected to the oil return pipes so that the oil return pipes can communicate with each other. Oil ON-OFF switching means are provided in each oil return pipe. The oil ON-OFF switching means serve both to ensure the flow of oil to the communication pipe from the oil separators and to turn on and shut off—by being opened and closed—the delivery of oil from each oil separator to the intake side of the respective compression mechanism. The pressure reducing means are provided in each oil return pipe or the communication pipe and serve to reduce the pressure of the oil sent from the oil separators to the communication pipe and the intake sides of the compressor mechanisms.

[0006] This compression mechanism oil equalizing circuit can supply the oil separated by the oil separators to the intake sides of the compression mechanisms by opening the oil ON-OFF switching means provided in the oil return pipes. Meanwhile, the supply of oil can be shut off by closing the oil ON-OFF switching means. As a result, the supply of oil to compression mechanisms that are stopped can be cut off and a sufficient amount of oil can be supplied to the compression mechanisms that are running, thereby improving the reliability of the oil supply to the compression mechanisms.

[0007] Claim 2 describes a compression mechanism oil equalizing circuit in accordance with claim 1, wherein the oil separators are provided so as to correspond to each compression mechanisms.

[0008] Claim 3 describes a compression mechanism oil equalizing circuit in accordance with claim 1 or 2, wherein the oil separators each include a first stage oil separator that is connected to the discharge side of the compression mechanism and configured to separate oil from the gaseous refrigerant and a second stage oil separator that is connected to the first stage oil separator and configured to collect the oil separated by the first stage oil separator. The oil return pipes are connected to the second stage oil separators.

[0009] With this compression mechanism oil equalizing circuit, the oil separated from the gaseous refrigerant in the first stage oil separator can be immediately sent to the second stage oil separator, thereby reducing the amount of oil mixing with the gaseous refrigerant flowing out of the first stage oil separator.

[0010] Claim 4 describes a compression mechanism oil equalizing circuit in accordance with claim 3, further provided with gas return pipes connecting the gas phase sections of the second stage oil separators to the intake sides of the compression mechanisms.

[0011] With this compression mechanism oil equalizing circuit, the gaseous refrigerant and other gaseous components mixed with the oil sent from the first stage oil separator and collected in the second stage oil separator can be returned to the intake side of the compression mechanism, thereby increasing the separating capacity of the oil separator as a whole.

[0012] Claim 5 describes a compression mechanism oil equalizing circuit in accordance with any one of claims 1 to 4, wherein the oil ON-OFF switching means are provided downstream of the parts where the oil return pipes connect to the communication pipe.

[0013] With this compression mechanism oil equalizing circuit, the oil ON-OFF switching means are provide downstream of the parts where the oil return pipes connect to the communication pipe. As a result, both the function of turning on and shutting off the delivery of oil to the intake side of the compression mechanism and the function of ensuring the flow of oil to the communication pipe from the oil separator can be accomplished with a single oil ON-OFF switching means, thereby reducing the number of parts making up the oil equalizing circuit.

[0014] Claim 6 describes a compression mechanism oil equalizing circuit in accordance with any one of claims 1 to 5, wherein the pressure reducing means are provided upstream of the parts where the oil return pipes connect to the communication pipe.

[0015] With this compression mechanism oil equalizing circuit, the pressure reducing means are provided upstream of the parts where the oil return pipes connect to the communication pipe. As a result, both the function of reducing the pressure of the oil flowing to the intake side of the compression mechanism and the function of reducing the pressure of the oil flowing toward the communication pipe can be accomplished with a single pressure reducing means, thereby reducing the number of parts making up the oil equalizing circuit.

[0016] Claim 7 describes a compression mechanism oil equalizing circuit in accordance with any one of claims 1 to 6, wherein the pressure reducing means are capillary tubes.

[0017] With this compression mechanism oil equalizing circuit, the structure is simplified because capillary tubes are used as the pressure reducing means.

[0018] The refrigeration system described in claim 8 is provided with a plurality of compression mechanisms for compressing the refrigerant, an oil equalizing circuit in accordance with any one of claims 1 to 7 for equalizing the distribution of oil among the compression mechanisms, and an oil equalization control means. The oil equalization control means detects if the compression mechanisms are running or stopped, executes control to close the oil ON-OFF switching means corresponding to the stopped compression mechanisms so that oil does not flow to the intake sides of the stopped compression mechanisms, and executes control to open the oil ON-OFF switching means corresponding to running compression mechanisms so that oil is supplied to the intake side of the running compression mechanisms.

[0019] Since this refrigeration system is provided with an oil equalization control means for controlling the oil equalizing circuit, oil can be supplied exclusively to the compression mechanisms that are running by opening and closing the oil ON-OFF switching means of the compression mechanisms in accordance with whether or not each compression mechanism is running. Thus, the reliability of the oil supply to the compression mechanisms can be improved.

[0020] The refrigeration system described in claim 9 is provided with a plurality of compression mechanisms for compressing the refrigerant, an oil equalizing circuit in accordance with any one of claims 1 to 7 for equalizing the distribution of oil among the compression mechanisms, and an oil equalization control means. The oil equalization control means detects if the compression mechanisms are running or stopped, executes control to close the oil ON-OFF switching means corresponding to the stopped compression mechanisms so that oil does not flow to the intake sides of the stopped compression mechanisms, and, when one compression mechanism is running, executes control to open the oil ON-OFF switching means corresponding to the running compression mechanism so that oil is supplied to the intake side of the running compression mechanism. When there are two or more compression mechanisms that are running, oil is supplied to the intake side of all of the compression mechanisms that are running by opening the oil ON-OFF switching means corresponding to the running compression mechanisms one at a time for a prescribed period of time each in a periodic manner, keeping the other oil ON-OFF switching means closed while one is open.

[0021] With this refrigeration mechanism, the oil equalization control means for controlling the oil equalizing circuit can achieve the following oil equalization operating modes. When one compression mechanism is running, the oil equalization control means supplies oil by opening only the oil ON-OFF switching means corresponding to the compression mechanism that is running. When two or more compression mechanisms are running, the oil equalization control means supplies oil to the compression mechanisms that are running by supplying oil to one running compression mechanism at a time for a prescribed period of time each in a periodic manner. When two or more compression mechanisms are running, oil is supplied intermittently to only one compression mechanism at a time through the oil return pipes and, as a result, oil is supplied to all of the running compression mechanisms with certainty. Thus, the reliability of the oil supply to the compression mechanisms can be improved.

[0022] Claim 10 describes a refrigeration system in accordance with claim 9, wherein the oil equalization control means controls the oil ON-OFF switching means corresponding to the compression mechanisms that are running in such a manner that when one oil ON-OFF switching means is switched from the open state to the closed state and another oil ON-OFF switching means is switched from the closed state to the open state, there is a transitional period during which both oil ON-OFF switching means are in the open state simultaneously.

[0023] With this refrigeration system, the oil equalization control means for controlling the oil equalizing circuit controls the oil ON-OFF switching means in such a manner that when one oil ON-OFF switching means is switched from the open state to the closed state and another oil ON-OFF switching means is switched from the closed state to the open state, there is a transitional period during which both oil ON-OFF switching means are in the open state simultaneously. Consequently, such undesirable situations as all of the oil ON-OFF switching means being closed such that the discharge of oil from the oil separators is obstructed can be prevented. As a result, the flow of oil through the oil equalizing circuit can be switched in a reliable manner.

[0024] The refrigeration system heat source unit described in claim 11 is a heat source unit for a refrigeration system provided with a compression mechanism for compressing the refrigerant, the heat source unit being provided with an oil separator, an oil return pipe, and a connection pipe. The oil separator is provided on the discharge side of the compression mechanism and serves to separate oil from the gaseous refrigerant. The oil return pipe is provided with an oil ON-OFF switching means that can turn on and shut off the supply of oil from the oil separator to the intake side of the compression mechanism. The connection pipe is connected to the oil return pipe between the oil separator and the oil ON-OFF switching means and makes it possible to connect to the oil return pipes of the compressor mechanisms of other heat source units.

[0025] With this refrigeration system heat source unit, when a plurality of heat source units are connected together in parallel and the oil ON-OFF switching means of the oil return pipes of the heat source units are opened, oil separated by the oil separators can be supplied to the intake side of the compression mechanisms of the heat source units. Meanwhile, the supply of oil to the intake sides of the compression mechanisms of the heat source units can be shut off by closing the oil ON-OFF switching means provided in the oil return pipes of the heat source units. As a result, the supply of oil to compression mechanisms that are stopped can be cut off and oil can be supplied exclusively to the compression mechanisms that are running, thereby improving the reliability of the oil supply to the compression mechanisms.

[0026] Claim 12 describes a refrigeration system heat source unit in accordance with claim 11, wherein the oil separator includes a first stage oil separator that is connected to the discharge side of the compression mechanism and configured to separate oil from the gaseous refrigerant and a second stage oil separator that is connected to the first stage oil separator and configured to collect the oil separated by the first stage oil separator. The oil return pipe is connected to the second stage oil separator.

[0027] With this refrigeration system heat source unit, the oil separated from the gaseous refrigerant in the first stage oil separator can be immediately sent to the second stage oil separator, thereby reducing the amount of oil mixing with the gaseous refrigerant flowing out of the first stage oil separator.

[0028] Claim 13 describes a refrigeration system heat source unit in accordance with claim 12, further provided with a gas return pipe connecting the gas phase section of the second stage oil separator to the intake side of the compression mechanism.

[0029] With this refrigeration system heat source unit, the gaseous refrigerant and other gaseous components mixed with the oil sent from the first stage oil separator and collected in the second stage oil separator can be returned to the intake side of the compression mechanism, thereby increasing the separating capacity of the oil separator as a whole.

[0030] The refrigeration system described in claim 14 is provided with a plurality of refrigeration system heat source units in accordance with any one of claims 11 to 13 and a communication pipe connecting the connection pipes of the heat source units together.

[0031] With this refrigeration system, oil can be equalized among the heat source units because the oil return pipes of the oil separators of the heat source units are connected together.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0032]FIG. 1 is a schematic view of the refrigerant circuit of an air conditioning system provided with an oil equalizing circuit in accordance with a first embodiment of the present invention.

[0033]FIG. 2 is illustrates the control states of the oil ON-OFF switching means in the oil equalizing circuit of the first embodiment.

[0034]FIG. 3 is a schematic view of the refrigerant circuit of an air conditioning system provided with an oil equalizing circuit in accordance with a second embodiment.

[0035]FIG. 4 is illustrates the refrigerant circuit and oil circuit of the compression mechanism inside a heat source unit of the second embodiment.

[0036]FIG. 5 is illustrates the connections between the oil equalizing unit and the compression mechanisms of the heat source units shown in FIG. 3.

[0037]FIG. 6 is a schematic view of the refrigerant circuit of an air conditioning system provided with an oil equalizing circuit in accordance with a third embodiment.

[0038]FIG. 7 is illustrates the refrigerant circuit and oil circuit of the compression mechanism inside a heat source unit of the third embodiment.

PREFERRED EMBODIMENTS OF THE INVENTION

[0039] Embodiments of compression mechanism oil equalizing circuits and refrigeration systems equipped therewith will now be described with reference to the drawings.

First Embodiment

[0040] (1) Constituent Features of the Refrigerant Circuit and Oil Equalizing Circuit of an Air Conditioning System

[0041]FIG. 1 is a schematic view of the refrigerant circuit of an air conditioning system 1 serving as a first embodiment of a compression mechanism oil equalizing circuit in accordance with the present invention and a refrigeration system provided with the same.

[0042] The air conditioning system 1 is provided with one heat source unit 2 and a plurality of user units 5 (two in this embodiment) connected in parallel thereto. It is used, for example, to air-condition an office building or the like. The heat source unit 2 is equipped chiefly with first, second, and third compressors 21 a to 21 c, a four-way selector valve 12, and heat-source-side heat exchanger 13. In this embodiment, the heat-source-side heat exchanger 13 serves to exchange heat between the refrigerant and air or water serving as a heat source. The user units 5 are each equipped chiefly with an expansion valve 14 and a user side heat exchanger 15. These devices 12 to 15 and 21 a to 21 c are connected together in sequence by refrigerant piping to form the refrigerant circuit of the air conditioning system 1.

[0043] The first, second, and third compressors 21 a to 21 c are compression mechanisms for compressing the gaseous refrigerant that returns to the heat source unit 2 after passing through the user-side heat exchanger 15 of the user unit 5 and are connected in parallel to form a compressor group 11. In this embodiment, the first compressor 21 a has a built-in inverter so that its operating capacity can be varied by controlling the rotational speed and the second and third compressors 21 b, 21 c are fixed-capacity compressors not provided with inverters.

[0044] The compressor group 11 is equipped with the following: first, second, and third compressors 21 a to 21 c; refrigerant intake main pipe 22, first, second, and third intake branch pipes 23 a to 23 c; an oil equalizing circuit 42; and a discharge merge pipe 31.

[0045] The refrigerant intake main pipe 22 connects to the outlet of the four-way selector valve 12. The discharge merge pipe 31 connects to the inlet of the four-way selector valve 12. The first, second, and third intake branch pipes 23 a to 23 c branch in a parallel manner from the refrigerant intake main pipe 22 and connect to the intake sides of the first, second, and third compressors 21 a to 21 c, respectively. The discharge sides of the first, second, and third compressors 21 a to 21 c are connected to the discharge merge pipe 31 through first, second, and third oil separators 24 a to 24 c (discussed later). The check valves 29 and 30 are provided downstream of the second and third oil separators 24 b, 24 c, respectively.

[0046] The oil equalizing circuit 42 serves to equalize the oil distribution among the first, second, and third compressors 21 a to 21 c and is provided with the following: first, second, and third oil separators 24 a to 24 c; first, second, and third oil return pipes 25 a to 25 c; a communication pipe 26; first, second, and third oil ON-OFF switching means 27 a to 27 c; and first, second, and third pressure reducing means 28 a to 28 c. The first, second, and third oil separators 24 a to 24 c are connected to the discharge sides of the first, second, and third compressors 21 a to 21 c and serve to separate oil from the gaseous refrigerant. The first, second, and third oil return pipes 25 a to 25 c connect first, second, and third oil separators 24 a to 24 c to the intake sides of the compressors 21 a to 21 c (more specifically, to the first, second, and third intake branch pipes 23 a to 23 c), respectively, and serve to deliver the oil separated by the first, second, and third oil separators 24 a to 24 c to the intake sides of the compressors 21 a to 21 c. The communication pipe 26 is connected to the oil return pipes 25 a to 25 c so that the oil return pipes 25 a to 25 c can communicate with each other. The first, second, and third oil ON-OFF switching means 27 a to 27 c are provided in the oil return pipes 25 a to 25 c, respectively, and serve both to ensure the flow of oil to the communication pipe 26 from the oil separators 24 a to 24 c and to turn on and shut off the delivery of oil from the first, second, and third oil separators 24 a to 24 c to the intake sides of the compressors 21 a to 21 c. More specifically, the first, second, and third oil ON-OFF switching means 27 a to 27 c are solenoid valves provided downstream of the parts where the oil return pipes 25 a to 25 c connect to the communication pipe 26. The first, second, and third pressure reducing means 28 a to 28 c are provided in the oil return pipes 25 a to 25 c or in the communication pipe 26 and serve to reduce the pressure of the oil that flows from the first, second, and third oil separators 24 a to 24 c to the intake sides of the compressors 21 a to 21 c and the communication pipe 26. More specifically, the first, second, and third pressure reducing means 28 a to 28 c are capillary tubes provided upstream of the parts where the oil return pipes 25 a to 25 c connect to the communication pipe 26.

[0047] The air conditioning system 1 is further provided with an oil equalization control means 41 that detects if the first, second, and third compressors 21 a to 21 c are running or stopped and opens and closes the first, second, and third oil ON-OFF switching means 27 a to 27 c accordingly. More specifically, the oil equalization control means 41 detects if the compressors 21 a to 21 c are running or stopped, executes control to close the oil ON-OFF switching means corresponding to the stopped compressors so that oil does not flow to the intake sides of the stopped compressors, and executes control to open the oil ON-OFF switching means corresponding to running compressors so that oil is supplied to the intake sides of the running compressors. In this embodiment, the oil equalization control means 41 is installed inside the heat source unit 2.

[0048] (2) Operation of the Air Conditioning System and the Oil Equalizing Circuit

[0049] The operation of the air conditioning system 1 and oil equalizing circuit 42 of this embodiment will now be described using FIGS. 1 to 2. FIG. 2 illustrates the control states of the oil ON-OFF switching means 27 a to 27 c in the oil equalizing circuit 42 according to the operating pattern of the compressors 21 a to 21 c.

[0050] [1] Partial Load Operation (First Compressor Running)

[0051] When the air conditioning system 1 is run, first the inverter-controllable first compressor 21 a is started. As a result, oil together with gaseous refrigerant flows from the refrigerant intake main pipe 22 into the first compressor 21 a through the first intake branch pipe 23 a. The gaseous refrigerant drawn into the first compressor 21 a is then compressed and discharged, after which it flows into the first oil separator 24 a. Since the gaseous refrigerant discharged from the first compressor 21 a contains excess oil, the excess oil is separated from the gaseous refrigerant by vapor-liquid separation in the first oil separator 24 a. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the first oil separator 24 a, flows into the discharge merge pipe 31, and circulates through the refrigerant circuit of the air conditioning system 1.

[0052] Meanwhile, the oil equalization control means 41 detects that the first compressor 21 a is running and the second and third compressors 21 b, 21 c are stopped and issues an open command to the first oil ON-OFF switching means 27 a and a close command to the second and third oil ON-OFF switching means 27 b and 27 c corresponding to the second and third compressors 21 b, 21 c, which are stopped. (See {circle over (1)} of FIG. 2.) As a result of this control, oil separated by the first oil separator 24 a is returned to the first intake branch pipe 23 a through the first oil return pipe 25 a and is drawn again into the first compressor 21 a along with gaseous refrigerant. Since the second and third compressors 21 b, 21 c are stopped, the oil pressure at the parts where the second and third oil return pipes 25 b, 25 c connect to the communication pipe 26 is held higher than the oil pressure at the part where the first oil return pipe 25 a connects to the communication pipe 26 and oil that has collected in the second and third oil separators 24 b, 24 c and the second and third oil return pipes 25 b, 25 c flows into the first oil return pipe 25 a through the communication pipe 26. In this way, when only the first compressor 21 a is run, oil inside the refrigerant circuit is supplied only to the first compressor 21 a.

[0053] [2] Partial Load Operation (First and Second Compressors Running)

[0054] If, after the first compressor 21 a is started, the second compressor 21 b is started in order to increase the operating load, gaseous refrigerant flowing through the refrigerant intake main pipe 22 will be drawn into both the first compressor 21 a and the second compressor 21 b. Similarly to the gaseous refrigerant drawn into the first compressor 21 a, the gaseous refrigerant drawn into the second compressor 21 b is then compressed and discharged, after which it flows into the second oil separator 24 b where the gaseous refrigerant and oil are separated by vapor-liquid separation. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the second oil separator 24 b, flows into the discharge merge pipe 31, merges with the gaseous refrigerant discharged from the first compressor 21 a, and circulates through the refrigerant circuit of the air conditioning system 1.

[0055] Meanwhile, the oil equalization control means 41 detects that the first and second compressors 21 a, 21 b are running and the third compressor 21 c is stopped and issues an open command to the first and second oil ON-OFF switching means 27 a, 27 b and a close command to the third oil ON-OFF switching means 27 c. Then, the oil equalization control means 41 controls the first and second oil ON-OFF switching means 27 a, 27 b corresponding to the first and second compressors 21 a, 21 b (which are running) such that they are repeatedly opened and closed alternately (periodically) for a prescribed time t1 each, i.e., first the first oil ON-OFF switching means 27 a is opened for a prescribed time t1 while the second oil ON-OFF switching means 27 b is closed and next the second oil ON-OFF switching means 27 b is opened for a prescribed time t1 while the first oil ON-OFF switching means 27 a is closed, and so on. Thus, oil is supplied to the intake sides of the two compressors that are running, i.e., the first and second compressors 21 a, 21 b (see ({circle over (2)} of FIG. 2). As a result, oil is delivered to the first and second compressors 21 a, 21 b with certainty, although intermittently. The prescribed time t1 is set to an amount of time that takes into account such factors as the time that oil resides inside the running compressors 21 a, 21 b so that oil deficiencies do not occur at the compressors 21 a, 21 b during the periods when oil is not supplied to them. Furthermore, the oil equalization control means 41 opens and closes the first and second oil ON-OFF switching means 27 a, 27 b in such a manner that when one oil ON-OFF switching means (27 a, for example) is switched from the open state to the closed state and the other oil ON-OFF switching means (27 b, for example) is switched from the closed state to the open state, there is a transitional period Δt1 during which both oil ON-OFF switching means 27 a, 27 b are in the open state simultaneously (see {circle over (2)} of FIG. 2). More specifically, when the oil equalization control means 41 switches from a state in which one of the first and second oil ON-OFF switching means (27 a or 27 b) is open and the other of the first and second oil ON-OFF switching means (27 b or 27 a) is closed to the opposite state in which the open and closed oil ON-OFF switching means are reversed, it first opens the first or second oil ON-OFF switching means (27 b or 27 a) that was closed while keeping the first or second oil ON-OFF switching means (27 a or 27 b) that was open in the open state. It then closes the first or second oil ON-OFF switching means (27 a or 27 b) that was open after the period Δt1 has elapsed (see {circle over (2)} of FIG. 2).

[0056] [3] Full Load Operation (First, Second, and Third Compressors Running)

[0057] If, after the second compressor 21 b is started, the third compressor 21 c is started in order to achieve full-load operation, gaseous refrigerant flowing through the refrigerant intake main pipe 22 will be drawn into the first, second, and third compressors 21 a to 21 c. Similarly to the gaseous refrigerant drawn into the first and second compressors 21 a, 21 b, the gaseous refrigerant drawn into the third compressor 21 c is compressed and discharged, after which it flows into the third oil separator 24 c where the gaseous refrigerant and excess oil are separated by vapor-liquid separation. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the third oil separator 24 c, flows into the discharge merge pipe 31, merges with the gaseous refrigerant discharged from the first and second compressors 21 a, 21 b, and circulates through the refrigerant circuit of the air conditioning system 1.

[0058] Meanwhile, the oil equalization control means 41 detects that the first, second, and third compressors 21 a to 21 c are running and issues open commands to the first, second, and third oil ON-OFF switching means 27 a to 27 c. Then, similarly to when two compressors, i.e., the first and second compressors 21 a and 21 b, are run, the oil equalization control means 41 controls the first, second and third oil ON-OFF switching means 27 a to 27 c such that they are repeatedly opened and closed alternately (periodically) for a prescribed time t2 each, i.e., first the first oil ON-OFF switching means 27 a is opened for a prescribed time t2 while the second and third oil ON-OFF switching means 27 b, 27 c are closed, next the second oil ON-OFF switching means 27 b is opened for a prescribed time t2 while the first and third oil ON-OFF switching means 27 a, 27 c are closed, then the third oil ON-OFF switching means 27 c is opened for a prescribed time t2 while the first and second oil ON-OFF switching means 27 a, 27 b are closed, and so on. As a result, oil is supplied to the intake sides of the compressors that are running, i.e., the first, second, and third compressors 21 a to 21 c (see {circle over (3)} of FIG. 2). As a result, oil is delivered to the first, second, and third compressors 21 a to 21 c with certainty, although intermittently. Similarly to the previously described time period t1, the prescribed time t2 is set to an amount of time that takes into account such factors as the time that oil resides inside the running compressors 21 a to 21 c so that oil deficiencies do not occur at the compressors 21 a to 21 c during the periods when oil is not supplied to them.

[0059] Furthermore, similarly to when two compressors, i.e., the first and second compressors 21 a and 21 b, are run, the oil equalization control means 41 opens and closes the first, second, and third oil ON-OFF switching means 27 a to 27 c in such a manner that when one oil ON-OFF switching means (27 a, 27 b, or 27 c) is switched from the open state to the closed state and another oil ON-OFF switching means (27 a, 27 b, or 27 c) is switched from the closed state to the open state, there is a transitional period Δt2 during which both oil ON-OFF switching means are in the open state simultaneously (see {circle over (3)} of FIG. 2).

[0060] (3) Characteristic Features of the Air Conditioning System and the Oil Equalizing Circuit

[0061] The oil equalizing circuit 42 of this embodiment has the following characteristic features.

[0062] [1] Improved Reliability of the Oil Supply to the Compressors

[0063] With the oil equalizing circuit 42 of this embodiment, oil separated by the first, second, and third oil separators 24 a to 24 c can be delivered to the intake sides of the compressors 21 a to 21 c by opening the first, second, and third oil ON-OFF switching means 27 a to 27 c provided in the first, second, and third oil return pipes 25 a to 25 c. The delivery of oil to the compressors 21 a to 21 c can also be shut off by closing the first, second, and third oil ON-OFF switching means 27 a to 27 c. Furthermore, since the oil return pipes 25 a to 25 c are connected together by the communication pipe 26, the oil in the oil return pipes whose oil ON-OFF switching means are closed flows through communication pipe 26 into the oil return pipes whose oil ON-OFF switching means are open. As a result, the supply of oil to compressors that are stopped can be cut off and a sufficient amount of oil can be supplied to the compressors that are running, thereby improving the reliability of the oil supply to the first, second, and third compressors 21 a to 21 c.

[0064] Also, since the oil return pipes 25 a to 25 c are connected to the communication pipe 26 at a position downstream of the pressure reducing means 28 a to 28 c, the oil pressure in oil return pipes whose oil ON-OFF switching means are closed is higher than the oil pressure in the oil return pipes whose oil ON-OFF switching means are open. As a result, oil that has collected in oil separators and oil return pipes whose oil ON-OFF switching means 27 a to 27 c are closed flows into the oil return pipes whose oil ON-OFF switching means are open through the communication pipe 26. As a result, oil is reliably delivered to the compressors that are running.

[0065] [2] Simply Constructed Oil Equalizing Circuit

[0066] In the oil equalizing circuit 42 of this embodiment, the first, second, and third oil ON-OFF switching means 27 a to 27 c are provided downstream of the parts where the oil return pipes 25 a to 25 c connect to the communication pipe 26. As a result, both the function of turning on and cutting off the delivery of oil to the intake side of the compressors 21 a to 21 c and the function of ensuring the flow of oil to the communication pipe 26 from the oil separators 24 a to 24 c can be accomplished, thereby reducing the number of parts making up the oil equalizing circuit 42.

[0067] Furthermore, with this oil equalizing circuit 42, the first, second, and third pressure reducing means 28 a to 28 c are provided upstream of the parts where the oil return pipes 25 a to 25 c connect to the communication pipe 26. As a result, both the function of reducing the pressure of the oil flowing to the intake side of the compressors 21 a to 21 c and the function of reducing the pressure of the oil flowing toward the communication pipe 26 can be accomplished, thereby reducing the number of parts making up the oil equalizing circuit 42. Also, the construction is simple because capillary tubes are used for the first, second, and third pressure reducing means 28 a to 28 c.

[0068] [3] Periodic Open/Close Control of the Oil ON-OFF Switching Means Corresponding to Compressors that are Running

[0069] With the air conditioning system 1 of this embodiment, the oil equalization control means 41 can accomplish the following oil equalization operating modes. For example, when the first compressor 21 a is running and the second and third compressors 21 b, 21 c are stopped (i.e., when the number of running compressors is one), oil can be supplied to the first compressor 21 a alone by opening only the first oil ON-OFF switching means 27 a (see {circle over (1)} of FIG. 2). When the first and second compressors 21 a, 21 b are run and the third compressor 21 c is stopped, oil is supplied to both of the running compressors 21 a, 21 b by periodically supplying oil to one or the other of the running first and second compressors 21 a, 21 b for a prescribed time t1 each. Similarly, when the first, second, and third compressors 21 a to 21 c are all run, oil is supplied to all of the running compressors 21 a to 21 c by periodically supplying oil to one or the other of the first, second, and third compressors 21 a to 21 c for a prescribed time t2 each. Thus, when two or more compressors are running, oil is supplied intermittently to only one compressor at a time through the oil return pipes and, as a result, oil is supplied reliably to all of the running compressors. Thus, the reliability of the oil supply to the compressors can be improved.

[0070] Additionally, the oil equalization control means 41 controls the oil ON-OFF switching means in such a manner that when one oil ON-OFF switching means is switched from the open state to the closed state and another oil ON-OFF switching means is switched from the closed state to the open state, there is a transitional period during which both oil ON-OFF switching means are in the open state simultaneously. Consequently, such undesirable situations as all of the oil ON-OFF switching means being closed such that the discharge of oil from the oil separators is obstructed can be prevented. As a result, the flow of oil through the oil equalizing circuit 42 can be switched in a reliable manner.

[0071] Moreover, since the oil equalization control means 41 operates the oil ON-OFF switching means 27 a to 27 c by detecting which of the first, second, and third compressors 21 a to 21 c are running, the same control can be accomplished regardless of which compressors among the first, second, and third compressors 21 a to 21 c are running. Thus, oil can be delivered reliably to two running compressors not only when it is the first and second compressors 21 a, 21 b that are running, but also when, for example, it is the first and third compressors 21 a, 21 c that are running. As a result, the service life of the compressors can be extended.

Second Embodiment

[0072] (1) Constituent Features of the Refrigerant Circuit and Oil Equalizing Circuit of the Air Conditioning System

[0073]FIG. 3 is a schematic view of the refrigerant circuit of an air conditioning system 101 serving as a second embodiment of a compression mechanism oil equalizing circuit in accordance with the present invention and a refrigeration system provided with the same. The air conditioning system 101 is provided with a plurality of heat source units (three in this embodiment), i.e., first, second, and third heat source units 102 a to 102 c, and a plurality of user units (not shown in the figure) connected in parallel to the heat source units. The air conditioning system 101 is a large-capacity air conditioning system in which the first, second, and third heat source units 102 a to 102 c are connected in parallel and each is provided with a plurality of compressors.

[0074] The first, second, and third heat source units 102 a to 102 c will now be described. Since the second and third heat source units 102 b, 102 c are constructed in the same manner as the first heat source unit 102 a, the first heat source unit 102 a will be described in detail while omitting similar descriptions of the second and third heat source units 102 b, 102 c.

[0075] The first heat source unit 102 a is equipped chiefly with a compression mechanism 111 a, a four-way selector valve 112, and heat-source-side heat exchanger 113. These devices 111 a, 112, 113 are connected together along with the user units (not shown) with refrigerant piping to form the refrigerant circuit of the air conditioning system 101.

[0076] The first compression mechanism 111 a serves to compress the gaseous refrigerant that returns to the heat source unit 102 a from the user unit (not shown) and is provided with the following: first, second, and third compressors 121 a, 122, 123; a refrigerant intake main pipe 124; first, second, and third intake branch pipes 125, 126, 127; first, second, and third oil separators 128 a, 129, 130; and first, second, and third oil return pipes 131 a, 132, 133. The refrigerant intake main pipe 124 connects to the outlet of the four-way selector valve 112. The refrigerant pipes at the outlets of the first, second, and third oil separators 128 a, 129, 130 merge with the discharge merge pipe 139. The discharge merge pipe 139 connects to the inlet of the four-way selector valve 112.

[0077] Among the first, second, and third compressors 121 a, 122, 123, the first compressor 121 a runs constantly when the heat source unit 102 a is running and the second and third compressors 122, 123 are started and stopped depending on the operating load of the first heat source unit 102 a.

[0078] The second intake branch pipe 126 branches from the refrigerant intake main pipe 124 and is connected such that it corresponds to the intake side of the second compressor 122. The third intake branch pipe 127 branches from the refrigerant intake main pipe 124 at a position downstream of the second intake branch pipe 126 and is connected such that it corresponds to the intake side of the third compressor 123. The first intake branch pipe 125 branches from the refrigerant intake main pipe 124 at a position downstream of the third intake branch pipe 127 and is connected to the intake side of the first compressor 121 a. The refrigerant intake main pipe 124 is arranged such that it slopes downward from the part where it connects to the second and third intake branch pipes 126, 127 toward the part where it connects to the first intake branch pipe 125.

[0079] The first, second, and third separators 128 a, 129, 130 are connected to the discharge sides of the respective first, second, and third compressors 121 a, 122, 123 in order to separate the oil from the gaseous refrigerant compressed by the first, second, and third compressors 121 a, 122, 123.

[0080] The first and second oil return pipes 131 a, 132 connect from the oil outlets of the first and second oil separators 128 a, 129 to the intake sides of the second and third compressors 122, 123, respectively. The third oil return pipe 133 is connected from the third oil separator 130 to the intake side of the first compressor 121a. More specifically, the first and second oil return pipes 131 a, 132 are connected to the second and third intake branch pipes 126, 127, respectively, and the third oil return pipe 133 is connected to the refrigerant intake main pipe 124 at a position downstream of the second intake branch pipe 126.

[0081] The first oil return pipe 131 a is connected to the intake side of the second compressor 122 such that oil is delivered to the refrigerant intake main pipe 124 due to gravity when the first compressor 121 a is running and the second and third compressors 122, 123 are stopped. The second oil return pipe 132 is connected to the intake side of the third compressor 123 such that oil is delivered to the refrigerant intake main pipe 124 due to gravity when the first and second compressors 121 a, 122 are running and the third compressor 123 is stopped. More specifically, the second and third intake branch pipes 126, 127 are arranged such that they slope downward from the part where they connect to the first and second oil return pipes 131 a, 132, respectively, toward the part where they connect to the refrigerant intake main pipe 124.

[0082] The second heat source unit 102 b has a second compressor mechanism 111 b, a first compressor 121 b, a first oil separator 128 b, and a first oil return pipe 131 b, similarly to the first heat source unit 102 a which has a first compressor mechanism 111 a, a first compressor 121 a, a first oil separator 128 a, and a first oil return pipe 131 a. Likewise, the third heat source unit 102 c has a third compressor mechanism 111 c, a first compressor 121 c, a first oil separator 128 c, and a first oil return pipe 131 c.

[0083] The air conditioning system 101 is further provided with an oil equalizing circuit 142 for equalizing the distribution of oil among the compressor mechanisms 111 a to 111 c of the heat source units 102 a to 102 c. The oil equalizing circuit 142 is made up of the following: the first oil separator 128 a and first return pipe 131 a of the first heat source unit 102 a; the first oil separator 128 b and first oil return pipe 131 b of the second heat source unit 102 b; the first oil separator 128 c and first oil return pipe 131 c of the third heat source unit 102 c; a communication pipe 134; first, second, and third oil ON-OFF switching means 135 a to 135 c; and first, second, and third pressure reducing means 136 a to 136 c.

[0084] The oil separators 128 a to 128 c are connected to the discharge sides of the first, second, and third compressors 121 a to 121 c of the first, second, and third compression mechanisms 111 a to 111 c, respectively, and serve to separate oil from the gaseous refrigerant. As described previously, the oil return pipes 131 a to 131 c connect the first oil separators 128 a to 128 c of the heat source units 102 a to 102 c to the intake sides of the compressors 121 a to 121 c, respectively, and serve to deliver oil separated by the oil separators 128 a to 128 c to the intake sides of the compressors 121 a to 121 c. The communication pipe 134 is connected to the oil return pipes 131 a to 131 c so that the oil return pipes 131 a to 131 c can communicate with each other. More specifically, connection pipes 137 a to 137 c for connecting to the communication pipe 134 are connected to the oil return pipes 131 a to 131 c between the oil separators 128 a to 128 c and the first, second, and third oil ON-OFF switching means 135 a to 135 c. The oil return pipes 131 a to 131 c of the heat source units 102 a to 102 c are connected together through these connection pipes 137 a to 137 c.

[0085] The first, second, and third oil ON-OFF switching means 135 a to 135 c are provided in the oil return pipes 131 a to 131 c, respectively, and serve both to ensure the flow of oil to the communication pipe 134 from the oil separators 128 a to 128 c and to turn on and shut off the delivery of oil from the oil separators 128 a to 128 c to the intake sides of the compressors 121 a to 121 c. More specifically, the first, second, and third oil ON-OFF switching means 135 a to 135 c are solenoid valves provided downstream of the parts where the oil return pipes 131 a to 131 c connect to the communication pipe 134.

[0086] The first, second, and third pressure reducing means 136 a to 136 c are provided in the oil return pipes 131 a to 131 c or in the communication pipe 134 and serve to reduce the pressure of the oil that flows from the oil separators 128 a to 128 c to the intake sides of the compressors 121 a to 121 c and the communication pipe 134. More specifically, the first, second, and third pressure reducing means 136 a to 136 c are capillary tubes provided upstream of the parts where the oil return pipes 131 a to 131 c connect to the communication pipe 134.

[0087] The air conditioning system 101 is further provided with an oil equalization control means 141 that detects if the compression mechanisms 111 a to 111 c (more specifically, first compressor 121 a of the first compression mechanism 111 a, the first compressor 121 b of the second compression mechanism 111 b, and the first compressor 121 c of the third compression mechanism 111 c) are running or stopped and opens and closes the first, second, and third oil ON-OFF switching means 135 a to 135 c accordingly. More specifically, the oil equalization control means 141 detects if each of the first, second, and third compressors 111 a to 111 c is running or stopped and executes control to close the oil ON-OFF switching means corresponding to the stopped compressors so that oil does not flow to the intake sides of the stopped compressors and control to open the oil ON-OFF switching means corresponding to running compressors so that oil is supplied to the intake sides of the running compressors. In this embodiment, the oil equalization control means 141 is built into the oil equalizing unit 140 along with the communication pipe 134 of the oil equalizing circuit 142.

[0088] (2) Operation of the Air Conditioning System and the Oil Equalizing Circuit

[0089] The operation of the air conditioning system 101 and oil equalizing circuit 142 of this embodiment will now be described using FIGS. 3 to 5. FIG. 4 illustrates the refrigerant circuit and oil circuit of the compression mechanism 111 a inside the first heat source unit 102 a. FIG. 5 is a simplified view showing only the compression mechanism and oil equalizing circuit portions of FIG. 3 and illustrating the connections between the compression mechanisms 111 a to 111 c of the heat source units 102 a to 102 c and the oil equalizing circuit 142.

[0090] [1] Partial Load Operation (First Heat Source Unit Running)

[0091] When only the first heat source unit 102 a of the air conditioning system 101 is run, the first compressor 121 a of the first the compression mechanism 111 a is started and oil together with gaseous refrigerant flows from the refrigerant intake main pipe 124 into the first compressor 121 a through the first intake branch pipe 125. The gaseous refrigerant drawn into the first compressor 121 a is then compressed and discharged, after which it flows into the first oil separator 128 a. Since the gaseous refrigerant discharged from the first compressor 121 a contains excess oil, the excess oil is separated from the gaseous refrigerant by vapor-liquid separation in the first oil separator 128 a. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the first oil separator 128 a and flows into the discharge merge pipe 139.

[0092] Meanwhile, the oil equalization control means 141 detects that the first compression mechanism 111 a of the first heat source unit 102 a (more specifically, the first compressor 121 a) is running and that the second compression mechanism 111 b of the second heat source unit 102 b and the third compression mechanism 111 c of the third heat source unit 102 c are stopped and issues an open command to the first oil ON-OFF switching means 135 a and a close command to the second and third oil ON-OFF switching means 135 b, 135 c. As a result of this control, similarly to the first embodiment, oil separated by the first oil separator 128 a is returned to the first intake branch pipe 125 through the first oil return pipe 131 a and drawn again into the first compression mechanism 111 a (more specifically, the first compressor 121 a) along with gaseous refrigerant. In this way, when only the first compression mechanism 111 a is run, oil is supplied to the first compression mechanism 111 a and not to the other compression mechanisms 111 b, 111 c.

[0093] In the heat source unit 102 a, the oil separated in the first oil separator 128 a leaves the oil outlet of the first oil separator 128 a, passes through the first oil return pipe 131 a, and flows into the second intake branch pipe 126. Then, since the second intake branch pipe 126 is configured so as to slope downward from the part where it connects to the first return pipe 131 a to the part where it connects to the refrigerant intake main pipe 124, the oil that flows into the second intake branch pipe 126 from the first oil return pipe 131 a flows down the second intake branch pipe 126 due to the action of gravity and into the refrigerant intake main pipe 124. After it flows into the refrigerant intake main pipe 124, the oil is drawn into the first compressor 121 a again along with gaseous refrigerant flowing through the refrigerant intake main pipe 124. Since the refrigerant intake main pipe 124 slopes downward toward the first intake branch pipe 125, the oil flowing into the refrigerant intake main pipe 124 flows readily toward the first intake branch pipe 125. In this way, an oil supply circuit is formed in which oil is supplied to the first compressor 121 a only.

[0094] If, after the first compressor 121 a is started, the second compressor 122 is started in order to increase the operating load of the first heat source unit 102 a, a portion of the gaseous refrigerant flowing through the refrigerant intake main pipe 124 will be drawn into the second compressor 122 through the second intake branch pipe 126. Meanwhile, the oil that flows into the second intake branch pipe 126 from the first oil return pipe 131 a is drawn into the second compressor 122 along with the gaseous refrigerant flowing through the second intake branch pipe 126. Similarly to the gaseous refrigerant drawn into the first compressor 121 a, the gaseous refrigerant drawn into the second compressor 122 is then compressed and discharged, after which it flows into the second oil separator 129 where the gaseous refrigerant and oil are separated by vapor-liquid separation. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the second oil separator 129 and flows into the discharge merge pipe 139.

[0095] In the first heat source unit 102 a, the oil separated in the second oil separator 129 leaves the oil outlet of the second oil separator 129, passes through the second oil return pipe 132, and flows into the third intake branch pipe 127. Then, since the third intake branch pipe 127, similarly to the second intake branch pipe 126, is configured so as to slope downward from the part where it connects to the second return pipe 132 to the part where it connects to the refrigerant intake main pipe 124, the oil that flows into the third intake branch pipe 127 from the second oil return pipe 132 flows into the refrigerant intake main pipe 124 due to the action of gravity. The third intake branch pipe 127 connects?? at a position closer to the first intake branch pipe 125 than the second intake branch pipe 126 does, i.e., at a position further downstream relative to the flow of the gaseous refrigerant. Consequently, the oil that flows into the refrigerant intake main pipe 124 from the third intake branch pipe 127 is drawn into the first compressor 121 a again along with the gaseous refrigerant flowing through the refrigerant intake main pipe 124 and does not flow into the second compressor 122. In this way, an oil supply circuit is formed inside the first compression mechanism 111 a such that oil is supplied in turn to the first and second compressors 121 a, 122 only.

[0096] If, after the second compressor 122 is started, the third compressor 123 is started in order to raise the first heat source unit 102 a to full-load operation, a portion of the gaseous refrigerant flowing through the refrigerant intake main pipe 124 will be drawn into the third compressor 123 through the third intake branch pipe 127. Meanwhile, the oil that flows into the third intake branch pipe 127 from the second oil return pipe 132 is drawn into the third compressor 123 along with the gaseous refrigerant flowing through the third intake branch pipe 127. Similarly to the gaseous refrigerant drawn into the first and second compressors 121 a and 122, the gaseous refrigerant drawn into the third compressor 123 is compressed and discharged, after which it is separated from the oil by vapor-liquid separation in the third oil separator 130. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the third oil separator 130 and flows into the discharge merge pipe 139.

[0097] Meanwhile, inside the first heat source unit 102 a, the oil separated in the third oil separator 130 leaves the oil outlet of the third oil separator 130, passes through the third oil return pipe 133, and flows into refrigerant intake main pipe 124 at a position between where the first intake branch pipe 125 connects and where the third intake branch pipe 127 connects. In this way, an oil supply circuit is formed in which oil is supplied in turn to all of the compressors, i.e., the first, second, and third compressors 121 a, 122, 123, of the first compression mechanism 111 a.

[0098] [2] Partial Load Operation (First and Second Heat Source Units Running)

[0099] Now a situation in which, after the first heat source unit 102 a is started, the second compression mechanism 111 b of the second heat source unit 102 b is started in order to further increase the operating load will be described. The operation of the compressors that make up the second compression mechanism 111 b is not described here because it is the same as the operation of the first compression mechanism 111 a.

[0100] When the first and second compression mechanisms 111 a, 111 b are run, the oil equalization control means 141 detects that the first and second compression mechanisms 111 a, 111 b are running and the third compression mechanism 111 c is stopped and issues an open command to the first and second oil ON-OFF switching means 135 a, 135 b and a close command to the third oil ON-OFF switching means 135 c. As a result of this control, the distribution of oil between the first and second compression mechanisms 111 a, 111 b is equalized similarly to the oil equalization control executed between the compressors 21 a and 21 b of the first embodiment and, thus, oil is supplied to the compression mechanisms that are running, i.e., the first and second compression mechanisms 111 a, 111 b.

[0101] [3] Partial Load Operation (First, Second, and Third Heat Source Units 102 a to 102 c Running)

[0102] Now a situation in which, after the second compression mechanism 111 b is started, the third compression mechanism 111 c of the third heat source unit 102 c is started in order to achieve full-load operation will be described. The operation of the compressors that make up the third compression mechanism 111 c is not described here because it is the same as the operation of the first and second compression mechanisms 111 a, 111 b.

[0103] When the compression mechanisms 111 a to 111 c are run, the oil equalization control means 141 detects that the compression mechanisms 111 a to 111 c are running and issues an open command to the first, second, and third oil ON-OFF switching means 135 a to 135 c. As a result of this control, the distribution of oil between the compression mechanisms 111 a to 111 c is equalized similarly to the oil equalization control executed between the compressors 21 a to 21 c of the first embodiment and, thus, oil is supplied to the compression mechanisms that are running, i.e., the compression mechanisms 111 a to 111 c.

[0104] (3) Characteristic Features of the Air Conditioning System and the Oil Equalizing Circuit

[0105] With the oil equalizing circuit 142 of this embodiment, the same oil equalization control as is executed by the oil equalizing circuit 42 of the first embodiment is executed among the heat source units 102 a to 102 c.

[0106] Thus, the reliability of the oil equalization control in a large-capacity air conditioning system provided with a plurality of heat source units can be improved by executing oil equalization control among the heat source units 102 a to 102 c.

[0107] Also, while in the past it has been necessary to produce a variety of heat source units range from small capacity to large capacity in order to meet the demands of users, a large-capacity air conditioning system having highly reliable oil equalization control can be achieved by connecting a plurality of small-capacity heat source units together with the previously described oil equalizing circuit. Thus, the need to produce heat source units of various capacities is eliminated and manufacturing costs can be reduced.

[0108] Also, since the heat source units 102 a to 102 c are each provided with connection pipes 137 a to 137 c serving as a pipe nozzle for connecting to the communication pipe 134, when the heat source units 102 a to 102 c are connected in parallel, the oil equalizing circuit 142 can be established with ease by merely connecting the communication pipe 134 to the connection pipes 137 a to 137 c.

[0109] Furthermore, onsite installation can be made easier by producing an oil equalizing unit 140 as a product that comprises an oil equalization control means 141 and a portion of an oil equalizing circuit 142 (i.e., communication pipe 134, etc.) like those previously described.

Third Embodiment

[0110] (1) Constituent Features of the Refrigerant Circuit and Oil Equalizing Circuit of the Air Conditioning System

[0111]FIG. 6 is a schematic view of the refrigerant circuit of an air conditioning system 201 serving as a third embodiment of a compression mechanism oil equalizing circuit in accordance with the present invention and a refrigeration system provided with the same. The air conditioning system 201 is provided with a plurality of heat source units (three in this embodiment), i.e., first, second, and third heat source units 202 a to 202 c, and a plurality of user units (not shown in the figure) connected in parallel to the heat source units. The air conditioning system 201 is a large-capacity air conditioning system in which, similarly to the air conditioning system 101 of the second embodiment, the first, second, and third heat source units 202 a to 202 c are connected in parallel and each is provided with a plurality of compressors.

[0112] The first, second, and third heat source units 202 a to 202 c will now be described. Since the second and third heat source units 202 b, 202 c are constructed in the same manner as the- first heat source unit 202 a, the first heat source unit 202 a will be described in detail while omitting similar descriptions of the second and third heat source units 202 b, 202 c (portions of the compression mechanisms of the second and third heat source units 202 b, 202 c have also been simplified in FIG. 6).

[0113] The first heat source unit 202 a is equipped chiefly with a first compression mechanism 211 a, a four-way selector valve 212, and heat-source-side heat exchanger 213. These devices 211 a, 212, 213 are connected together along with the user units (not shown) by refrigerant piping to form the refrigerant circuit of the air conditioning system 201.

[0114] As shown in FIGS. 6 and 7, the first compression mechanism 211 a serves to compress the gaseous refrigerant that returns to the heat source unit 202 a from the user units (not shown) and is provided with the following: first, second, and third compressors 221 a, 222, 223; a refrigerant intake main pipe 224; first, second, and third intake branch pipes 225, 226, 227; first, second, and third oil separators 228 a, 229, 230; and first, second, and third separator-side oil return pipes 231 a, 232, 233. The refrigerant intake main pipe 224 connects to the outlet of the four-way selector valve 212. The refrigerant pipes at the outlets of the first, second, and third oil separators 228 a, 229, 230 merge with the discharge merge pipe 239. The discharge merge pipe 239 connects to the inlet of the four-way selector valve 212.

[0115] Among the first, second, and third compressors 221 a, 222, 223, the first compressor 221 a runs constantly when the heat source unit 202 a is running and the second and third compressors 222, 223 are started and stopped depending on the operating load of the first heat source unit 202 a.

[0116] The second intake branch pipe 226 branches from the refrigerant intake main pipe 224 and is connected such that it corresponds to the intake side of the second compressor 222. The first intake branch pipe 225 and the third intake branch pipe 227 branch from the refrigerant intake main pipe 224 at positions farther downstream than the position where the second intake branch pipe 226 branches from the same and connect so as to correspond to the intake sides of the first compressor 221 a and third compressor 223, respectively. The first, second, and third intake branch pipes 225, 226, 227 are configured such that they slope downward toward the parts where they connect to the refrigerant intake main pipe 224.

[0117] The first, second, and third oil separators 228 a, 229, 230 are connected to the discharge sides of the respective first, second, and third compressors 221 a, 222, 223 in order to separate the oil from the gaseous refrigerant compressed by the first, second, and third compressors 221 a, 222, 223. The oil separator 228 a is designed to separate oil in two stages and has a first stage oil separator 251 a that is connected to the discharge side of the first compressor 221 a and configured to separate oil from the gaseous refrigerant and a second stage oil separator 252 a that is connected to the first stage oil separator 251 a and configured to collect the oil separated by the first stage oil separator 251 a. With this first oil separator 228 a, the oil separated from the gaseous refrigerant in the first stage oil separator 251 a can be immediately sent to the second stage oil separator 252 a, thereby reducing the amount of oil mixing with the gaseous refrigerant flowing out of the first stage oil separator 251 a.

[0118] The separator-side oil return pipe 231 a is connected from the oil outlet of the first oil separator 228 a to the intake side of the second compressor 222. More specifically, the separator-side oil return pipe 231 a is connected to the second stage oil separator 252 a of the first oil separator 228 a. More specifically, the separator-side oil return pipe 231 a has a separator-side oil ON-OFF switching means 253 a, a separator-side non-return means 254 a, and a separator-side pressure reducing means 255 a. The separator-side oil ON-OFF switching means 253 a is a solenoid valve for turning on and shutting off the supply of oil from the first oil separator 228 a to the intake side of the compressor 222. The separator-side non-return means 254 a is a check valve that only allows oil to flow from the first oil separator 228 a to the intake side of the compressor 222. The separator-side pressure reducing means 255 a is a capillary tube for reducing the pressure of the oil flowing from the first oil separator 228 a to the intake side of the second compressor 222.

[0119] The second stage oil separator 252 a of the first oil separator 228 a is provided with a gas return pipe 259 a that connects the gas phase section inside the second stage oil separator 252 a to the intake side of the second compressor 222. As a result, gaseous refrigerant and other gaseous components mixed with the oil sent from the first stage oil separator 251 a and collected in the second stage oil separator 252 a can be returned to the intake side of the second compressor 222. The gas return pipe 259 a has a pressure reducing means 260 a comprising a capillary tube for reducing the pressure of the oil flowing from the second stage oil separator 252 a to the intake side of the second compressor 222.

[0120] The separator-side oil return pipe 232 is connected from the oil outlet of the second oil separator 229 to the intake side of the third compressor 223. The separator-side oil return pipe 232 has a separator-side pressure reducing means 237 comprising a capillary tube for reducing the pressure of the oil flowing from the second oil separator 229 to the intake side of the third compressor 223.

[0121] The separator-side oil return pipe 233 is connected from the third oil separator 230 to the intake side of the first compressor 221 a. The separator-side oil return pipe 233 has a separator-side pressure reducing means 238 comprising a capillary tube for reducing the pressure of the oil flowing from the third oil separator 230 to the intake side of the first compressor 221 a.

[0122] The separator-side oil return pipe 231 a is connected to the intake side of the second compressor 222 such that oil is delivered to the refrigerant intake main pipe 224 by gravity when the first compressor 221 a is running and the second and third compressors 222, 223 are stopped. The separator-side oil return pipe 232 is connected to the intake side of the third compressor 223 such that oil is delivered to the refrigerant intake main pipe 224 by gravity when the first and second compressors 221 a, 222 are running and the third compressor 223 is stopped. More specifically, the second and third intake branch pipes 226, 227 are arranged such that they slope downward from the parts where they connect to the separator-side oil return pipes 231 a, 232, respectively, toward the parts where they connect to the refrigerant intake main pipe 224.

[0123] Similarly to the first heat source unit 202 a, which is provided with a first compression mechanism 211 a including a first compressor 221 a, a first oil separator 228 a comprising a first stage oil separator 251 a and a second stage oil separator 252 a, and a separator-side oil return pipe 231 a having a gas return pipe 259 a and a separator side oil ON-OFF switching means 253 a, the second heat source unit 202 b is provided with a second compression mechanism 211 b including a first compressor 221 b, a first oil separator 228 b comprising a first stage oil separator 251 b and a second stage oil separator 252 b, and a separator-side oil return pipe 231 b having a gas return pipe 259 b and a separator side oil ON-OFF switching means 253 b. Likewise, the third heat source unit 202 c is provided with a third compression mechanism 211 c including a first compressor 221 c, a first oil separator 228 c comprising a first stage oil separator 251 c and a second stage oil separator 252 c, and a separator-side oil return pipe 231 c having a gas return pipe 259 c and a separator side oil ON-OFF switching means 253 c.

[0124] The air conditioning system 201 is further provided with an oil equalizing circuit 242 for equalizing the distribution of oil among the heat source units 202 a to 202 c. The oil equalizing circuit 242 is made up of the following: a communication pipe 234; the first oil separator 228 a, oil-equalization-side oil return pipe 235 a, and connection pipe 236 a of the first heat source unit 202 a; the first oil separator 228 b, oil-equalization-side oil return pipe 235 b, and connection pipe 236 b of the second heat source unit 202 b; the first oil separator 228 c, oil-equalization-side oil return pipe 235 c, and connection pipe 236 c of the third heat source unit 202 c.

[0125] The oil-equalization-side oil return pipes 235 a to 235 c are provided in the heat source units 202 a to 202 c, respectively, and serve to connect the second stage oil separators 252 a to 252 c of the heat source units 202 a to 202 c to the intake sides of the first compressors 221 a to 221 c, respectively, so that oil separated by the first oil separators 228 a to 228 c is delivered to the intake sides of the compression mechanisms 211 a to 211 c (more specifically, the refrigerant intake main pipe). More specifically, the oil-equalization-side oil return pipes 235 a to 235 c are connected the second stage oil separators 252 a to 252 c of the first oil separators 228 a to 228 c of the heat source units 202 a to 202 c and include the following: oil-equalization-side pressure reducing means 256 a to 256 c; first, second and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c; and oil-equalization-side non-return means 258 a to 258 c.

[0126] The communication pipe 234 is connected to the oil-equalization-side oil return pipes 235 a to 235 c so that the oil-equalization-side oil return pipes 235 a to 235 c can communicate with each other. More specifically, connection pipes 236 a to 236 c are connected to the oil-equalization-purpose oil return pipes 235 a to 235 c between the first oil separators 228 a to 228 c and the oil-equalization-side oil ON-OFF switching means 257 a to 257 c, respectively. By connecting the connection pipes 236 a to 236 c to the communication pipe 234, the oil-equalization-side oil return pipes 235 a to 235 c of the heat source units 202 a to 202 c are allowed to communicate with each other.

[0127] The oil-equalization-side pressure reducing means 256 a to 256 c are provided in the oil-equalization-side oil return pipes 235 a to 235 c and serve to reduce the pressure of the oil that flows from the first oil separators 228 a to 228 c to the intake sides of the compression mechanisms 211 a to 211 c and the communication pipe 234. More specifically, the oil-equalization-side pressure reducing means 256 a to 256 c are capillary tubes provided between the second stage oil separators 252 a to 252 c and the connection pipes 236 a to 236 c.

[0128] The first, second, and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c are provided in the oil-equalization-side oil return pipes 235 a to 235 c, respectively, and serve both to ensure the flow of oil to the communication pipe 234 from the first oil separators 228 a to 228 c and to turn on and shut off the delivery of oil from the first oil separators 228 a to 228 c to the intake sides of the compression mechanisms 211 a to 211 c. More specifically, the first, second, and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c are solenoid valves provided downstream of the parts where the oil-equalization-side oil return pipes 235 a to 235 c connect to the communication pipe 234.

[0129] The oil-equalization-side non-return means 258 a to 258 c are check valves that only allow oil to flow from the first oil separators 228 a to 228 c to the intake sides of the compression mechanisms 211 a to 211 c.

[0130] The air conditioning system 201 is further provided with an oil equalization control means 241 that detects if the compression mechanisms 211 a to 211 c (more specifically, the first compressor 221 a of the first compression mechanism 211 a, the first compressor 221 b of the second compression mechanism 211 b, and the first compressor 221 c of the third compression mechanism 211 c) are running or stopped and opens and closes the first, second, and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c accordingly. More specifically, the oil equalization control means 241 detects if the first, second, and third compression mechanisms 211 a to 211 c is running or stopped and executes control to close the oil ON-OFF switching means corresponding to the stopped compressors so that oil does not flow to the intake sides of the stopped compressors and control to open the oil ON-OFF switching means corresponding to running compressors so that oil is supplied to the intake sides of the running compressors.

[0131] (2) Operation of the Air Conditioning System and the Oil Equalizing Circuit

[0132] The operation of the air conditioning system 201 and oil equalizing circuit 242 of this embodiment will now be described using FIGS. 6 to 7.

[0133] [1] Partial Load Operation (First Heat Source Unit Running)

[0134] When only the first heat source unit 202 a of the air conditioning system 201 is run, the first compressor 221 a of the compression mechanism 211 a is started and oil together with gaseous refrigerant flows from the refrigerant intake main pipe 224 into the first compressor 221 a through the first intake branch pipe 225. The gaseous refrigerant drawn into the first compressor 221 a is then compressed and discharged, after which it flows into the first oil separator 228 a. Since the gaseous refrigerant discharged from the first compressor 221 a contains excess oil, the excess oil is separated from the gaseous refrigerant by vapor-liquid separation in the first oil separator 228 a. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the first oil separator 228 a and flows into the discharge merge pipe 239. Since the first oil separator 228 a is made up of a first stage oil separator 251 a and a second stage oil separator 252 a, the oil separated from the gaseous refrigerant in the first stage oil separator 251 a can be immediately sent to the second stage oil separator 252 a, thereby reducing the amount of oil mixing with the gaseous refrigerant flowing out of the first stage oil separator 251 a.

[0135] Meanwhile, the oil equalization control means 241 detects that the first compression mechanism 211 a of the first heat source unit 202 a (more specifically, the first compressor 221 a) is running and that the second compression mechanism 211 b of the second heat source unit 202 b and the third compression mechanism 211 c of the third heat source unit 202 c are stopped and issues an open command to the first oil-equalization-side oil ON-OFF switching means 257 a and a close command to the second and third oil-equalization-side oil ON-OFF switching means 257 b, 257 c. As a result of this control, oil separated by the first oil separator 228 a is returned to the intake side of the first compression mechanism 211 a through the oil-equalization-side oil return pipe 235 a and is drawn again into the first compression mechanism 211 a (more specifically, the first compressor 221 a) along with gaseous refrigerant. In this way, when only the first compression mechanism 211 a is run, oil is supplied to the first compression mechanism 211 a and not to the second and third compression mechanisms 211 b, 211 c.

[0136] In addition to issuing open and close commands to the first, second, and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c as just described, the oil equalization control means 241 issues an open command to the separator-side oil ON-OFF switching means 253 a of the first heat source unit 202 a, which is running, and a close command to the separator-side oil ON-OFF switching means 253 b, 253 c of second and third heat source units, which are stopped. As a result, oil that has collected inside the oil separators 228 b, 228 c (more specifically, the second stage oil separators 252 b, 252 c) of the stopped heat source units 202 b, 202 c is sent to the oil-equalization-side oil return pipe 235 a of the first heat source unit 202 a through the oil-equalization-side return pipes 235 b, 235 c and the communication pipe 234 and delivered to the intake side of the first compression mechanism 211 a of the first heat source unit 202 a. Thus, oil does not collect at the intake side of the compression mechanisms 211 b, 211 c of the stopped heat source units 202 b, 202 c.

[0137] In the heat source unit 202 a, the oil sent to the second stage oil separator 252 a of the first oil separator 228 a is temporarily collected in the second stage oil separator 252 a. Then, the gaseous refrigerant and other gaseous components mixed with the oil are sent to the intake side of the second compressor 222 through the gas return pipe 259 a and the other liquid components are sent to the intake side of the second compressor 222 through the separator-side oil return pipe 231 a because the separator-side oil ON-OFF switching means 253 a is open. The oil that flows into the second intake branch pipe 226 from the second stage oil separator 252 a through the separator-side oil return pipe 231 a descends through the second intake branch pipe 226 due to the action of gravity and is delivered to the refrigerant intake main pipe 224. After it flows into the refrigerant intake main pipe 224, the oil joins oil delivered from the other compression mechanisms 211 b, 211 c through the communication pipe 234 and is drawn into the first compressor 221 a again along with gaseous refrigerant flowing through the refrigerant intake main pipe 224.

[0138] If, after the first compressor 221 a is started, the second compressor 222 is started in order to increase the operating load of the first heat source unit 202 a, a portion of the gaseous refrigerant flowing through the refrigerant intake main pipe 224 will be drawn into both the second compressor 222 through the second intake branch pipe 226. The oil that flows into the second intake branch pipe 226 from the gas return pipe 259 a and the separator-side oil return pipe 231 a is drawn into the second compressor 222 along with the gaseous refrigerant flowing through the second intake branch pipe 226. Similarly to the gaseous refrigerant drawn into the first compressor 221 a, the gaseous refrigerant drawn into the second compressor 222 is then compressed and discharged, after which it is separated from the oil by vapor-liquid separation in the second oil separator 229. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the second oil separator 229 and flows into the discharge merge pipe 239.

[0139] In the first heat source unit 202 a, the oil separated in the second oil separator 229 leaves the oil outlet of the second oil separator 229, passes through the second oil return pipe 232, and flows into the third intake branch pipe 227. As a result, the oil that flows into the third intake branch pipe 227 from the separator-side oil return pipe 232 is delivered to the refrigerant intake main pipe 224 due to the action of gravity. The oil that flows into the refrigerant intake main pipe 224 from the third intake branch pipe 227 is drawn into the first compressor 221 a again along with the gaseous refrigerant flowing through the refrigerant intake main pipe 224 and does not flow into the second compressor 222.

[0140] If, after the second compressor 222 is started, the third compressor 223 is started in order to raise the first heat source unit 202 a to full-load operation, a portion of the gaseous refrigerant flowing through the refrigerant intake main pipe 224 will be drawn into the third compressor 223 through the third intake branch pipe 227. The oil that flows into the third intake branch pipe 227 from the separator-side oil return pipe 232 is drawn into the third compressor 223 along with the gaseous refrigerant flowing through the third intake branch pipe 227. Similarly to the gaseous refrigerant drawn into the second compressor 222, the gaseous refrigerant drawn into the third compressor 223 is then compressed and discharged, after which it is separated from the oil by vapor-liquid separation in the second oil separator 230. Then, the gaseous refrigerant passes through the refrigerant pipe at the outlet of the third oil separator 230 and flows into the discharge merge pipe 239.

[0141] In the first heat source unit 202 a, the oil separated in the third oil separator 230 leaves the oil outlet of the third oil separator 230, passes through the separator-side oil return pipe 233, and flows into the first intake branch pipe 225. In this way, oil is supplied in turn to all of the first, second, and third compressors 221 a, 222, 223 of the first compression mechanism 211 a.

[0142] [2] Partial Load Operation (First and Second Heat Source Units Running)

[0143] Now a situation in which, after the first heat source unit 202 a is started, the second compression mechanism 211 b of the second heat source unit 202 b is started in order to further increase the operating load will be described. The operation of the compressors that make up the second compression mechanism 211 b is not described here because it is the same as the operation of the first compression mechanism 211 a.

[0144] When the first and second compression mechanisms 211 a, 211 b are run, the oil equalization control means 241 detects that the first and second compression mechanisms 211 a, 211 b are running and the third compression mechanism 211 c is stopped and issues an open command to the first and second oil-equalization-side oil ON-OFF switching means 257 a, 257 b and a close command to the third oil-equalization-side oil ON-OFF switching means 257 c. The oil equalization control means 241 issues an open command to the separator-side oil ON-OFF switching means 253 a, 253 b of the first and second heat source units 202 a, 202 b, which are running. As a result of this control, the distribution of oil between the first and second compression mechanisms 211 a, 211 b is equalized similarly to the oil equalization control executed between the compressors 21 a and 21 b of the first embodiment and, thus, oil is supplied to the compression mechanisms that are running, i.e., the first and second compression mechanisms 211 a, 211 b.

[0145] [3] Full Load Operation (First, Second, and Third Heat Source Units Running)

[0146] Now a situation in which, after the second compression mechanism 211 b is started, the third compression mechanism 211 c of the third heat source unit 202 c is started in order to achieve full-load operation will be described. The operation of the compressors that make up the third compression mechanism 211 c is not described here because it is the same as the operation of the first and second compression mechanisms 211 a, 211 b.

[0147] When the compression mechanisms 211 a to 211 c are run, the oil equalization control means 241 detects that the compression mechanisms 211 a to 211 c are running and issues open commands both to the first, second, and third oil-equalization-side oil ON-OFF switching means 257 a to 257 c as well as to the separator-side oil ON-OFF switching means 253 a to 253 c of the first, second, and third heat source units 202 a to 202 c, which are running. As a result of this control, the distribution of oil between the compression mechanisms 211 a to 211 c is equalized similarly to the oil equalization control executed between the compressors 21 a to 21 c of the first embodiment and, thus, oil is supplied to the compression mechanisms that are running, i.e., the compression mechanisms 211 a to 211 c.

[0148] (3) Characteristic Features of the Air Conditioning System and the Oil Equalizing Circuit

[0149] With the oil equalizing circuit 242 of this embodiment, the same oil equalization control as is executed by the oil equalizing circuit 142 of the second embodiment is executed among the heat source units 202 a to 202 c and the same effects as the second embodiment are achieved.

[0150] Also with the oil equalization circuit of this embodiment, the separating capacity of the oil separator as a whole is improved because the first oil separators 228 a to 228 c of the heat source units 202 a to 202 c are two-stage oil separators comprising a first stage oil separator 251 a to 251 c and a second stage oil separator 252 a to 252 c.

Other Embodiments

[0151] Although embodiments of the present invention have been described herein with reference to the drawings, the specific constituent features are not limited to those of these embodiments and variations can be made within a scope that does not deviate from the gist of the invention.

[0152] (1) It is also acceptable to provide a common oil separator of the kind described in the first embodiment with respect to a plurality of compressors.

[0153] (2) The number of compressors and the type of compressor are not limited to the first, second, and third embodiments. Also, the number of heat source units and the operating capacity of the heat source units are not limited to the second and third embodiments.

[0154] (3) In the second embodiment, the communication pipe that connects the heat source units together and the oil equalization control means are provided as separate units from the heat source units. It is also acceptable for these items to be built into the heat source units.

[0155] Applicability to Industry

[0156] By utilizing the present invention, the supply of oil to compression mechanisms that are stopped can be cut off and a sufficient amount of oil can be supplied to the compression mechanisms that are running, thereby improving the reliability of the oil supply to the compression mechanisms. 

1. An oil equalizing circuit comprising: a plurality of oil separators that are configured to be provided on discharge sides of compression mechanisms that compress refrigerant in a vapor compression refrigeration system and configured to separate oil from the refrigerant; a plurality of oil return pipes that are configured to connect each of the oil separators to an intake side of one of the compression mechanisms and configured to allow the oil separated by the oil separators to be delivered to the intake sides of the compression mechanisms; a communication pipe connected to the oil return pipes so that the oil return pipes can communicate with each other; a plurality of oil ON-OFF switching mechanisms that are provided in the oil return pipes and configured and arranged both to ensure a flow of the oil from the oil separators to the communication pipe and to open and close such that the ON-OFF switching mechanisms turn on and shut off a delivery of the oil from the oil separators to the intake sides of the compression mechanisms; and a plurality of pressure reducing mechanisms that are provided in one of the oil return pipes and the communication pipe and configured to reduce pressure of the oil sent from the oil separators to the communication pipe and the intake sides of the compressor mechanisms.
 2. The oil equalizing circuit as recited in claim 1, wherein the oil separators are provided in such a manner that one of the oil separators corresponds to each of the compression mechanisms.
 3. The oil equalizing circuit as recited in claim 1, wherein the oil separators each include a first stage oil separator that is configured to be connected to one of the discharge sides of the compression mechanisms and configured to separate the oil from the refrigerant and a second stage oil separator that is connected to the first stage oil separator and configured to collect the oil separated by the first stage oil separator; and the oil return pipes are connected to the second stage oil separators.
 4. The oil equalizing circuit as recited in claim 3, further comprising a plurality of gas return pipes configured to connect gas phase sections of the second stage oil separators to the intake sides of the compression mechanisms.
 5. The oil equalizing circuit as recited in claim 1, wherein the oil ON-OFF switching mechanisms are provided downstream of the parts where the oil return pipes connect to the communication pipe.
 6. The oil equalizing circuit as recited in claim 1, wherein the pressure reducing mechanisms are provided upstream of parts where the oil return pipes connect to the communication pipe.
 7. The oil equalizing circuit as recited in claim 1, wherein the pressure reducing mechanisms are capillary tubes.
 8. A refrigeration system comprising: a plurality of compression mechanisms configured to compress refrigerant; an oil equalizing circuit including a plurality of oil separators that are provided on discharge sides of the compression mechanisms and configured to separate oil from the refrigerant; a plurality of oil return pipes that connect each of the oil separators to an intake side of one of the compression mechanisms and are configured to allow the oil separated by the oil separators to be delivered to the intake sides of the compression mechanisms; a communication pipe connected to the oil return pipes so that the oil return pipes can communicate with each other; a plurality of oil ON-OFF switching mechanisms that are provided in the oil return pipes and configured and arranged to ensure a flow of the oil from the oil separators to the communication pipe and to open and close such that the ON-OFF switching mechanisms turn on and shut off a delivery of the oil from the oil separators to the intake sides of the compression mechanisms; and a plurality of pressure reducing mechanisms that are provided in one of the oil return pipes and the communication pipe and configured to reduce pressure of the oil sent from the oil separators to the communication pipe and the intake sides of the compressor mechanisms; and an oil equalization control mechanism configured to detect if the compression mechanisms are running or stopped, execute control to close the oil ON-OFF switching mechanisms corresponding to the stopped compression mechanisms so that the oil does not flow to the intake sides of the stopped compression mechanisms, and execute control to open the oil ON-OFF switching mechanisms corresponding to running compression mechanisms so that the oil is supplied to the intake sides of the running compression mechanisms.
 9. A refrigeration system comprising: a plurality of compression mechanisms configured to compress refrigerant; an oil equalizing circuit including a plurality of oil separators that are provided on discharge sides of the compression mechanisms and configured to separate oil from the refrigerant; a plurality of oil return pipes that connect each of the oil separators to an intake side of one of the compression mechanisms and are configured to allow the oil separated by the oil separators to be delivered to the intake sides of the compression mechanisms; a communication pipe connected to the oil return pipes so that the oil return pipes can communicate with each other; a plurality of oil ON-OFF switching mechanisms that are provided in the oil return pipes and configured and arranged to ensure a flow of the oil from the oil separators to the communication pipe and to open and close such that the ON-OFF switching mechanisms turn on and shut off a delivery of the oil from the oil separators to the intake sides of the compression mechanisms; and a plurality of pressure reducing mechanisms that are provided in one of the oil return pipes and the communication pipe and configured to reduce pressure of the oil sent from the oil separators to the communication pipe and the intake sides of the compressor mechanisms; and an oil equalization control mechanism configured to detect if the compression mechanisms are running or stopped; execute control to close the oil ON-OFF switching mechanisms corresponding to the stopped compression mechanisms so that the oil does not flow to the intake sides of the stopped compression mechanisms; and execute control such that when one of the compression mechanisms is running, the oil is supplied to the intake side of the running compression mechanism by opening the oil ON-OFF switching mechanism corresponding to the running compression mechanism and when at least two of the compression mechanisms are running, the oil is supplied to the intake sides of the at least two of the running compression mechanisms by opening the oil ON-OFF switching mechanisms corresponding to the at least two of the running compression mechanisms one at a time for a prescribed period of time in a periodic manner, while keeping the other of the oil ON-OFF switching mechanisms closed.
 10. The refrigeration system as recited in claim 9, wherein the oil equalization control mechanism controls the oil ON-OFF switching mechanisms corresponding to the compression mechanisms that are running in such a manner that when one of the oil ON-OFF switching mechanisms is switched from a open state to a closed state and another of the oil ON-OFF switching mechanisms is switched from a closed state to a open state, there is a transitional period during which the one of the oil ON-OFF switching mechanisms and the another of the oil ON-OFF switching mechanisms are both in the open states, respectively simultaneously.
 11. A heat source unit comprising: a compression mechanism configured to compress refrigerant; an oil separator that is disposed on a discharge side of the compression mechanism and configured to separate oil from the refrigerant; an oil return pipe provided with an oil ON-OFF switching mechanism that can turn on and shut off the supply of the oil from the oil separator to an intake side of the compression mechanism; a connection pipe that is connected to the oil return pipe between the oil separator and the oil ON-OFF switching mechanism and configured to connect the heat source unit to oil return pipes of the compressor mechanisms of other heat source units.
 12. The heat source unit as recited in claim 11, wherein the oil separator includes a first stage oil separator that is connected to the discharge side of the compression mechanism and configured to separate oil from the refrigerant and a second stage oil separator that is connected to the first stage oil separator and configured to collect the oil separated by the first stage oil separator; and the oil return pipe is connected to the second stage oil separator.
 13. The heat source unit recited in claim 12, further comprising a gas return pipe connecting a gas phase section of the second stage oil separator to the intake side of the compression mechanism.
 14. A refrigeration system comprising: a plurality of heat source units as recited in claim 11; and a communication pipe connecting the connection pipes of the heat source units together.
 15. The oil equalizing circuit as recited in claim 2, wherein the oil separators each include a first stage oil separator that is configured to be connected to one of the discharge sides of the compression mechanisms and configured to separate the oil from the refrigerant and a second stage oil separator that is connected to the first stage oil separator and configured to collect the oil separated by the first stage oil separator; and the oil return pipes are connected to the second stage oil separators.
 16. The oil equalizing circuit as recited in claim 15, further comprising a plurality of gas return pipes configured to connect gas phase sections of the second stage oil separators to the intake sides of the compression mechanisms.
 17. The oil equalizing circuit as recited in claim 16, wherein the oil ON-OFF switching mechanisms are provided downstream of parts where the oil return pipes connect to the communication pipe.
 18. The oil equalizing circuit as recited in claim 17, wherein the pressure reducing mechanisms are provided upstream of the parts where the oil return pipes connect to the communication pipe.
 19. The oil equalizing circuit as recited in claim 18, wherein the pressure reducing mechanisms are capillary tubes.
 20. A refrigeration system comprising: a plurality of heat source units as recited in claim 13; and a communication pipe connecting the connection pipes of the heat source units together. 